The Nobel Prize Laureate Who Disappeared

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The Nobel Prize Laureate who disappeared

How a man from Örebro was awarded the Nobel Prize thanks to a lucky eye for design and great attention to detail. Assistant and Reader

Manne Siegbahn, born in 1886 in Örebro, was registered as a student at Lund University at the age of 19 and began studying physics. In the same year he was appointed teaching assistant and a few years later assistant to Janne Rydberg. He turned out to have a great talent for physics and gained a PhD in 1911 at the age of 25 with a thesis on methods of mea- suring magnetic ﬁelds. Manne Siegbahn was soon given on a leading position at the department. Rydberg was prone to illness and, Siegbahn substituted for him.

Manne Siegbahn 1886 - 1978

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Rydberg’s successor 35

At that time there were many unsolved mysteries in physics research. The discovery of X-rays and radioactivity and the work of Planck and Einstein led the research onto new lines. After completing his PhD in 1911, Siegbahn fo- cused his research on X-rays. During the summer vacations he travelled to leading departments of physics around Europe – Göttingen, Munich, Heidelberg, Paris and Berlin – to acquaint himself with current work. When Rydberg retired in 1919, Siegbahn was internationally renowned as one of the leaders in his ﬁeld and he was appointed directly as Rydberg’s successor without having to apply for the position. Mysterious rays

On one of his European trips, Siegbahn visited Professor Wilhelm Röntgen. Just over ten years previously, Röntgen had reported the discovery of a mysterious, penetrating type of ray. On the top ﬂoor of the Department of Physics in Würzburg, Röntgen had his private quarters, and on the ground ﬂoor was the department laboratory. On the afternoon of 8 November 1895, Röntgen went down to his laboratory in Würzburg and here he describes what he saw:

[...] The vacuum tube is surrounded by a fairly close-fitting shield of black paper; it is then possible to see, in a completely darkened room, that paper covered on one side with barium pla- tinocyanide lights up with brilliant fluorescence when brought into the neighbourhood of the tube, whether the painted side or the other be turned towards the tube. The fluorescence is still visible at two metres distance. In the early experiments, X-rays were generated in glass discharge tubes. The radiation was emitted from the anode (A on the drawing).

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X-rays 37

Soon Röntgen could show that the rays not only penetrated paper screens, but also various types of material. He sent his report of these results to around 100 colleagues as a New Year’s greeting and soon after it was being cited in the world press. The discovery of X-rays led to intensive research around the world and Wilhelm Röntgen was awarded the ﬁrst Nobel Prize for Physics in 1901.

The ﬁrst medical X-ray image, taken by Professor Wilhelm Röntgen of his wife Anna Bertha Ludwig’s hand with wedding ring. Wavelength measurements

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Bragg’s law 39

Sir William Henry Bragg and William Lawrence Bragg

Since X-rays behave like a wave motion, their wavelength can be measured. In 1913 Sir William Henry Bragg and William Lawrence Bragg (father and son) showed how the reﬂection in a crystal could be used to measure wavelengths. The formula for this came to be known as Bragg’s law. They received the 1915 Nobel Prize for Physics for this discovery. Using Bragg’s law it became possible to determine the wavelength of X-rays. In the shadow of the war

In 1914 Englishman Henry Moseley had discovered a fundamental connection between atomic number and wavelength in the X-ray spectra of various elements. He had found that if the root of the frequency ν or √(1/ λ) is plotted against the element’s ordinal number in the periodic table, a straight line is produced. Using this type of diagram and with the help of Niels Bohr’s theories, Moseley was able to draw the conclusion that the atomic number and the charge number of the nucleus, Ζ, were the same number. Atomic numbers became meaningful. In August 1915 Moseley was killed in action at Gallipoli.

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X-ray tubes and spectrometers 41

In Lund, Manne Siegbahn had followed the deve- lopments in the new research ﬁeld that opened up. Siegbahn realised that Moseley’s work should be continued and expanded to more elements and other wavelength regions. Spectrometers with the entire radiation path in a vacuum made it possible to observe spectra on longer wavelengths than previously. With a new method, Siegbahn was also able to increase the accuracy of the measurements by over 100 times. With higher resolution, many new components were discovered in the groups of lines that had previously been observed. Designers and instrument makers

One of Siegbahn’s pupils, Arvid Leide, wrote that the successes were due to:

… Siegbahn’s personal qualities as an in- genious and inspiring teacher, his scientiﬁc intuition and his lucky eye for design.

X-ray tubes, spectrometers and vacuum pumps were manufactured at the department, initially by caretaker A L Pedersen and later by a specially employed precision instrument maker A S Ahlström.

Alfred S Ahlström, had his workshop on Stora Fiskaregatan 8 in a dark room in the yard. The same room contained workshop, storeroom, kitchen and bed. He had introduced a ﬂat rate and all repairs cost 1 krona and 25 öre regardless of how long they took to carry out. If the repair was particularly amusing or interesting then the work was free.

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New discoveries 43

Manne Siegbahn had a rare ability to attract talented doctoral students. In the years 1914 – 1925, no fewer than 15 doctoral theses were published, several of which were of epoch- making signiﬁcance. The projects comprised systematic studies and precision measurements of X-ray spectra throughout the periodic table. Two groups of spectral lines had previously been observed in each element, called the K series and the L series. Using the new spectrometers, it was found that the L series contained many more lines than had previously been Siegbahns own photographic spectra with observed. In 1916 Siegbahn discovered a new the L series in four different elements. group of lines at longer wavelengths, which became known as the M series.

A new measurement method and the precision scale engraved on the lower part of the spectro- meter, gave the great improvement in accuracy. Bohr’s model of the atom

In 1913 Niels Bohr had presented his model of the atom. With this model it was possible in principle to explain how the characteristic X-ray lines came about. In the X-ray tube, an electron is knocked out of an inner electron shell. The space is ﬁlled by an electron from an outer shell and the surplus energy is emitted as an X-ray. Bohr’s model could also be used to explain the connection between wavelengths and atomic numbers discovered by Moseley. However, the many new spectral lines observed with Siegbahn’s high-resolution spectrometers could not be explained.

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Sommerfeld’s ellipses 45

In order to improve the model of the atom, Arnold Sommerfeld (German mathemati- cian and theoretical physicist) assumed that the electrons moved in elliptical trajectories around the atomic nucleus instead of in Bohr’s circular trajectories. Sommerfeld’s elliptical electron trajectories assumed that many of the X-ray lines – as observed – were divided into a number of components. Now precision measurements were needed that could only be performed in Lund. In order to access accurate data, Arnold Sommerfeld corresponded with Manne Siegbahn.The eyes of the atomic physi- cists were on Lund.

In a general presentation that Siegbahn held in autumn 1918, he said: Based on our precise measurements, Sommerfeld has proved that his formula is generally correct. Measurements now exist that make it possible to check the value of his formula even more precisely. International attention

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New effects 47 Nobel Prize

In 1922, the chair of physics in Uppsala became vacant, and, as previously in Lund, Siegbahn was oﬀered the post without having to apply. He accepted and left Lund in 1923. In 1925 Manne Siegbahn was awarded the dormant 1924 Nobel Prize for Physics for his discoveries and research in the ﬁeld of X-ray spectroscopy. In 1936 the Royal Swedish Academy of Sciences established a research institute for physics in Stockholm and Manne Siegbahn was appointed head of the institute. The primary focus of the research there was on nuclear physics.

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